Basics

Bluetooth Low Energy (Bluetooth LE) was introduced in 2010 in Bluetooth Core Specification Version 4.0. Bluetooth Smart is the brand name for the Bluetooth Low Energy introduced in Bluetooth Core Specification Version 4.1.

The hierarchy of the Bluetooth LE is physical channel, physical link, logical transport, logical link and L2CAP channel

Physical Layer

Bluetooth LE radio can have either a receiver, transmitter or both thus allowing for the establishment of either simplex or duplex communication.

Bluetooth LE operates in the 2.4 GHz ISM band at 2400-2483.5 MHz. The LE system uses 40 RF channels. These RF channels have center frequencies 2402 + k * 2 MHz, where k = 0, ..., 39.

Radio power in Bluetooth LE varies between -20 dBm and 10 dBm.

• Physical Channel

In Bluetooth LE a device can be either a master or a slave. A physical link is formed between the master and each slave. Direct links between slaves are not supported. Each slave is permitted to have a link to more than one master. Devices are permitted to be both master and slave at the same time. Role changes between a master and slave device are not supported.

• Physical Link

The physical link is used as a transport for one or more logical links. Logical links support asyncronus trafic. Traffic on logical links is multiplexed onto the physical link assigned by a scheduling function in the resource manager.

Link Layer

Each Bluetooth LE device can be either a Slave or Master and one device can be a slave to several masters.

Each Bluetooth LE device must have either a unique identifier a random identifier or both. Unique device identifiers are 28 bit long. The least significant 24 bits are the company_assigned and the 24 most significant bits contain the company_id.

The Bluetooth LE specification defines 40 RF channels: three channels for discovery, connection initiation and broadcasting and 37 channels for user data transfer. Each channel is allocated a unique channel index.

• Logical Link

Signaling and user data are both transmitted over logical links. The controll protocol used at this layer is called Link Layer Protocol (LL). Devices that are active in a piconet have a default LE asynchronous connection logical transport (LE ACL). It is used to transport the LL protocol signaling. The default LE ACL is the one created whenever a device joins a piconet. The Link Layer function uses the LL protocol to control the operation of devices in the piconet and provide services to manage the lower architectural layers.

• L2CAP layer

Just as in Bluetooth classic (BR/EDR), above the link layer is the L2CAP layer. It provides a channel based abstraction to applications and services. It is responsible for fragmentation and de-fragmentation of application data as well as multiplexing and de-multiplexing of multiple channels over a shared logical link. L2CAP has a protocol control channel that is carried over the primary ACL logical transport. In addition to L2CAP, LE provides two additional protocol layers that reside on top of L2CAP. The SecurityManager protocol (SMP) uses a fixed L2CAP channel to implement the security functions between devices. The other one is the Attribute protocol (ATT). It provides a method for communicating small amounts of data over a fixed L2CAP channel. The Attribute protocol is also used by devices to determine the services and capabilities of other devices.

Security Overview

Bluetooth uses the Secure Simple Pairing mechanism with the following four paring modes:

• Numeric Comparison - This scheme is developed for scenarios where both devices are capable of displaying a six digit number and of having the user enter yes or no. The obvious use case is when a phone is paired with a computer. Both devices display a six decimal digit number and the user is asked to check if those match. This mode is supposed to eliminate the threat of man in the middle attacks.
• Just Works - It is designed for scenarios in which one of the devices is not capable of displaying six digits or accepting user input. A typical use case is pairing a mobile phone and a hands free device. The Just works mode uses the numeric comparison protocol, but the user is never shown a number, instead he is just asked to confirm the paring. This mode provides protection against passive eavesdropping but not against man in the middle attacks.
• Out of Band (OOB) - This mechanism uses an OOB communication such as NFC for exchanging cryptographic information, Bluetooth device address, etc.. The level of security of this paring mode is dependent on the protection that the used OOB protocol provides.
• Passkey Entry - This association mode is used when the device is capable of displaying six digits and the other have input capabilities. An example is the PC and keyboard use case, when the user is asked to enter the six symbols displayed on the screen.

Bluetooth uses the following Cryptographic primitives and algorithms:

• P-192 and P-256 (since v. 4.1) elliptic curves.
• Elliptic Curve Diffie-Hellman (DH76).
• E1, E21 and E22 algorithms based on SAFER+.
• E0 encryption algorithm based on the Massey-Rueppel scheme.
• SHA 256 and HMAC-SHA-256 (since v. 2.1)
• AES-CTR (Counter) and AES-CCM (Counter with CBC-MAC) algorithms (since v. 4.1).